The bombardment of metal surfaces with so-called abrasive materials is finding an increasing number of technical applications in recent years. Techniques such as grit blasting, shot blasting, sand blasting, shot peening and micro abrasion fall under this category of surface treatment technique. In each of these techniques, generally, an abrasive material, shot or grit, is mixed with a fluid and delivered at high velocity to impinge the surface to be treated. The technique used to deliver the abrasive material can be classified as wet or dry depending on the choice of fluid medium used to deliver the abrasive to the surface, usually water and air respectively. The generic term “abrasive bombardment” is used to refer to all such techniques in this specification.
Applications of these technologies include metal cutting, cold working metallic surfaces to induce desirable strain characteristics and the pre-treatment of surfaces to induce desirable texture (surface roughness) for the purposes of enhanced adhesion of further coating materials. (See Solomon et al., Welding research, 2003. October: p. 278-287; Momber et al., Tribology International, 2002. 35: p. 271-281; Arola et al., J. Biomed. Mat. Res., 2000. 53(5): p. 536-546; and Arola and Hall, Machining science and technology, 2004. 8(2): p. 171-192). An example of the latter is to be found in the biomedical sector where titanium implants are grit blasted with alumina or silica to achieve an optimum level of surface roughness that will maximize the adhesion of plasma sprayed hydroxyapatite (HA) coatings on the surface of the implants. HA coated implants are desirable because of the biomimetic properties of the apatite layer but an optimum bonding strength between the titanium surface and the apatite layer is also necessary.
It has been known for some time that during the bombardment of these surfaces some of the abrasive material becomes impregnated in the surface of the metal itself, which has generated some interest in these techniques as possible candidates for modifying surface chemistry in general. (See Arola et al. and Arola and Hall, supra). Again with reference to the biomedical sector one study has looked at shot blasting as a means of putting a hydroxyapatite layer directly on to a titanium surface in an effort to bypass the costly plasma spray process. Ishikawa, K., et al., Blast coating method: new method of coating titanium surface with Hydroxyapatite at room temperature. J. Biomed. Mat. Res., 1997. 38: p. 129-134. In this study, HA of an unspecified particle size distribution was used as the abrasive. However, given that the deposited layer of apatite could be removed with a benign washing regime it seems that a strong bond with the surface of the metal was not achieved.
Choi et al. (KR20030078480) refer to the use of a single calcium phosphate particle as a grit blasting media for the purposes of embedding the grit in the surface of dental implants but particle in excess of 190 μm are disclosed.
U.S. Pat. No. 6,502,442 ([6]) refers to the use of sintered HA as the abrasive using water as the fluid medium. Some impregnation of the HA was achieved in this instance as the HA was thermally processed.
Muller et al. (US2004158330) disclosed blasting particles comprising calcium phosphate contained in a glassy matrix. Other disclosures (e.g., U.S. Pat. Nos. 4,752,457 and 6,210,715) describe methods for the manufacture of calcium phosphate micro-spheres usually comprising a polymer component and complex methods of manufacturing the same, but their effectiveness as blasting media was not elucidated.
The Rocatec™ system for the silicization of metallic and other surfaces also uses individual particles having multiple components. This technology is used extensively in the dental arena. In this instance an alumina particle having an outer adherent layer of silica is propelled at a pre-roughened surface and upon impact the local heat generated in the vicinity of the impact causes the shattered silica outer layer to become fused to the surface a process referred to as ceramicization.
Bru-Magniez et al. (U.S. Pat. No. 6,431,958) have disclosed hard abrasive materials with multiple stratified layers for use in blasting abrasive bombardment techniques to modify surfaces. In this instance the purpose of the process was to embed or otherwise attach the stratified layer around the abrasive particles to the surface being treated. The outer layer comprises at least one polymer while the core ceramic material of choice is an oxide, carbide, nitride, or carbonitride.
The use of multiple stratified polymeric layers has been proposed. Lange et al. (U.S. Pat. No. 6,468,658) have disclosed a particle composed of a core base material and an outer adherent layer of titanium dioxide for blasting purposes
Further applications of abrasive bombardment for the purposes of surface modification are to be found in the biomedical sector such as for example the use of micro abrasion to clean the oxide slag from the struts of laser machined coronary stents and the impregnation of the surfaces of pacemakers and defibrillators with silica to increase the adhesion of further polymer coatings to the device.
A commonality among these examples is the use of a single type of solid particle in the fluid stream.
The recent significant interest in surface modification technology as it relates to biomedical devices is fueled by the success of the Drug Eluting Stent (DES). Since the introduction of endovascular techniques in the 1990's revascularisation strategies have changed dramatically over the last number of years. However, in-stent restenosis (ISR) remains a problem wherein rupture of the vessel lining at the stent site can cause platelet activation, the secretion of inflammation mediators and eventually smooth muscle cell (SMC) formation, a process analogous to scar formation around a wound site. Furthermore as the stent also contacts the blood it should not induce a foreign body reaction (FBR) in the tissue or blood cells, i.e., it should be biocompatible. The DES uses surface modification technology to combat these problems wherein the surface of the stent is used to deliver active agents (anti-restenosis and anti-thrombosis agents) usually in a polymer matrix locally to the device site where they are most needed. This technology was pioneered by Cordis with there Cypher stent which received FDA approval in 2003. Since then a number of other DES have appeared on the market all aimed at reducing ISR and thrombosis in patients that have percutaneous coronary intervention (PCI) procedures. All of these active devices use a polymer matrix to carry the drug on the surface of the stent and control its elution characteristics in vivo.
However problems have arisen with the DES attributed to a number of factors, among them, achieving proper control of the elution characteristics of the drug(s). The polymer matrix (which degrades with time to release the drug and the polymer degradation products) has been identified as a possible culprit in patients with hypersensitivity. Thus, there are continuing efforts to develop new methods to control the delivery and elution of the drugs.
A large body of prior art in the stent arena has been directed towards achieving passive coatings on the stent surface to mediate ISR. These include such processes as nitriding and carbon-nitriding, the use of carbon and silicon carbide coatings as well as processes to thicken or augment the native oxide layer on the surface of the stent materials including oxidation, ion implantation and electrochemical treatments such as electropolishing or electroplating with inert metals. All such processes however have a number of disadvantages and no one treatment technique as such provides the ideal surface for optimal clinical results.
Another arena of relevance is the area of biofilm formation at the surfaces of implantable devices wherein bacteria at the surface of implant surfaces arrange themselves into films with three dimensional macroscopic structure. In this instance the film itself can represent a barrier to standard antimicrobial treatments such as for example the systemic use of antibiotics. It is reported that the systemic dose of antibiotic required to kill bacterial biofilm infections can be up to 1000 times the systemic dose required to kill their planktonic counterparts in suspension often inducing unwanted and serious side effects in patients. Localized drug delivery at the surfaces of implantable devices has been mentioned as one method to target antimicrobial agents at the implant surface where they are most needed, preventing biofilm formation with the added advantage of using much lower dose rates than systemic treatments.
Currently most bactericidal strategies for localized drug delivery use polymer coatings or polymer micro spheres embedded in other suitable carrier matrices as carriers for antibacterial agents. In addition calcium phosphate salts including hydroxyapatite have been proposed as suitable carriers for antibiotics. Biomimetic deposition has been used to deposit nano crystalline apatite layers on the surfaces of orthopedic metallic implants that can then be loaded with drugs precipitated onto the inorganic coating from solution in a separate step (US20040131754). Such strategies can have dual advantage as for example in the arena of orthopedic implants where the calcium phosphate salt provides an osteoconductive benefit at the surface inducing bone in-growth in vivo while the antibiotic reduces the risk of biofilm formation, both factors contributing heavily to the need for revision procedures. However this approach is limited by the available surface area at the surface of the implant as this determines the amount of antibiotic that can be loaded. Furthermore the approach is multi-step as often the attachment of the ceramic layer involves high temperature (as for example in the case of plasma sprayed calcium phosphate coatings) or the attachment of the drug requires precise control of the pH and other process parameters precluding the simultaneous attachment of the inorganic salt and the antibacterial agent. Among the antibiotics that have been attached to metal surfaces via such methods are gentamycin, tobramycin, vancomycin, ampicillin, and others.
The range of therapeutic agents that could provide benefit for patients if present at the surface of implants is not limited to antibiotics or immuno-suppressants. Several studies have focused on placing other therapeutic agents at the surface of implantable devices to induce desirable in vivo responses. For example, some studies have focused on placing the functional molecules involved in these cascades at the surfaces of the implants. These include for example proteins among them hormones, growth factors, structural proteins, immunogens and antigens. As a corollary of this much work has focused on the design of peptides and proteins that have structural similarity to the active sites of the proteins involved in biological pathways. For example the use of RGD peptides in orthopedic applications, or bactericidal peptides have been proposed as strategies for combating bacterial infection in instances, e.g., where the bacteria have high resistance to conventional antibiotics.
As medical implants are increasingly tailored to the needs of the patient they can also be viewed as a means to deliver therapeutic agents for the treatment of other more patient specific diseases for example diabetes, cancers and other diseases not directly related to the primary function of the implant. An in vivo device lends itself to multiple functions wherein the surface of the device becomes a vehicle to deliver therapeutic agents that might be required to treat other diseases the patient may have.
The limiting factors in achieving therapeutic agent delivery capacity at the surfaces of implants generally surround the engineering and processing aspects. Methods to put these agents on the surface are required that are commensurate with maintaining the activity and structural integrity of the agents themselves and controlling the surface chemistry particularly there elution kinetics in vivo. As many of the agents desired are biological in nature, temperature and solution parameters such as pH etc can present barriers to realizing the benefit of the above mentioned surface modification strategies.
Surface modification of implant surfaces is not limited to the field of therapeutic agent delivery alone. In many cases surface modification of the implantable device may be required for the purposes of tailoring the physical properties of the surface such as, for example, in titanium based devices used in coronary intervention procedures, and in the treatment of pathological calcifications such as kidney stones. It would, however, be desirable to have devices with higher radio-opacity than that currently associated with these devices in vitro. This would facilitate their radiographic or even magnetic resonance imaging externally and dispense with the need for invasive procedures or endoscopes currently used with minimally invasive procedures. Examples include the doping of nitinol alloys with tertiary heavy elements such as platinum, palladium or tungsten among others to increase the radio opacity of the resulting alloy for biomedical and other applications (U.S. Pat. Nos. 7,128,757, 6,776,795, and 6,569,194).